The present invention relates to the field of molding thermoset compositions, and more particularly, to a method and system for a mold to improve the properties of release agents.
Applications for optical urethane resins have grown over the past fifteen years. The main reason for this growth results from the ability to tailor the resin formulation to impart a wide range of properties such as high glass transition temperature, chemical resistance, high light transmission, high refractive index, and low casting shrinkage. In addition, the urethane reactants can be selected to be compatible with a wide range of specialty additives.
Thermosetting urethanes are generally produced by cell-casting techniques. Organic resin materials are mixed until a uniform solution is obtained. The uniform mixture is then poured into a cell (or a mold), placed in an oven and cured at elevated temperatures. After the cure cycle, the mold is opened and the resulting plastic material is separated from the mold.
For optical applications, it is common to use glass as the casting surface. For casting large area, flat plastic sheets, float glass provides the necessary optical surface finish at an economical price. The casting process replicates the smooth optical finish of the glass mold surface. Unfortunately, urethane resins tenaciously adhere to silicate glass surfaces. Such adhesion damages the plastic material and glass inserts during the plastic/glass separation process. As a result, a mold release agent is typically used to inhibit adhesion between the urethane resin and the glass surface. To date, there have been various methods and materials to serve as the releasing agent. One broad class of releasing agent are silicones or other oils. These oils are applied to the glass surface as lubricating agents to provide a single casting release to the urethane. These materials have several drawbacks. Since these oils do not bond to the glass, they must be reapplied to the glass surface prior to each casting. The uniform application of these oils tends to be an art form and dependent upon the skill level of the individual operator. As a result, the release properties are inconsistent. In addition, the oil contaminants can be transferred to the plastic material thereby compromising the adhesion of thin films deposited onto the cast urethane sheet. Furthermore, certain oils can leave a hard to clean residue on the mold surface that can damage the optical surface finish of the mold after multiple castings. Both the number of surface defects and the cost of cleaning the mold surface increases with silicone oil residue buildup on the glass surfaces. Finally, if the mold surface has three dimensional features, such as prisms or lenses, the silicone oils can disrupt the sharp features necessary for the desired refractive or diffractive property.
Semi-permanent mold releases address some of the drawbacks described above. U.S. Pat. No. 5,204,126, assigned to the Nanofilm Corporation, describes a mold release for optical casting applications that uses a siloxane based, film forming amphiphilic molecule. U.S. Pat. No. 5,804,674, assigned to Daikin Industries, describes the application of a multicomponent film forming compound including silanes (for example, fluorinated trialkoxy silanes) and fluorinated olefins. The fluorinated alkyl chain imparts a lubricated, non-wettable, low energy surface to the glass insert that impedes the urethane from adhering to the glass surface. However, these semi-permanent mold releases also have drawbacks. Namely, semi-permanent coatings are subject to degradation after multiple castings. In addition, the dense film structure depends upon intensive cleaning and surface preparation of the glass surface. For example, some have tried treating the glass in acidic solutions, basic solutions, or plasma and UV/OZONE exposures. The film quality and durability depends on a uniformly clean and sensitized surface. In practice, the surface preparation quality is judged by the number of castings one obtains from a single treatment. Despite the achievement of multiple castings, the films tend to exhibit inconsistent lifetime properties. This is not a good attribute in a production environment where predictability is an important aspect of manufacturing. Many semi-permanent mold releases contain a sacrificial lubricant that facilitates the release properties. However, the lubricant can also contaminate the surface of the plastic. The contaminant can be removed by polishing techniques, wet chemistry cleaning, or plasma treatment. However, such cleaning techniques add cost and restricts the casting dimension one can process. The larger the casting, the more expensive the capital equipment necessary to post-clean the plastic window. In addition, the inventors have found it necessary to replenish the lubricant after each casting by rubbing the lubricant onto the glass surface. The process of wiping the lubricant is very sensitive to the skill of the operator. Too much lubricant imparts surface defects in the casting while too little lubricant results in damaged plastic or molds. Even if applied correctly, replenishing the lubricant may increase the lifetime of the mold, but it also increases the cost of the mold cleaning and maintenance. Other patents describe internal mold releases (hereinafter xe2x80x9cIMR""sxe2x80x9d) for urethanes. IMR formulations are numerous and include carboxylic acids, esters, fluorinated alcohols, phosphate esters and their complex salts. U.S. Pat. No. 5,753,730, assigned to The Mitsui Toatsu Chemical Company, describes the use of nonionic fluorinated and silicone based surfactants, as well as quaternary ammonium salts as IMR""s for urethane ophthalmic lens castings. U.S. Pat. No. 5,962,561 to Essilor describes using mixtures of phosphate esters as IMR""s for urethane ophthalmic lens castings. The inventors of the present invention have obtained only dissatisfying results when following the examples described in these patents. To date, the release properties on glass surfaces have been poor at best and more typically results in either the glass mold or the plastic casting breaking. Therefore, there is a need for a better release system and methods that will solve the problems mentioned in the foregoing paragraphs.
The present invention provides a method and a system for fabricating articles made from thermoset resins using an ionic mold release agent. The system in the present invention uses a mold having a metal oxide surface which will enhance the release characteristics of the mold release agent. The release agent can be applied internally with the resin composition or externally on the metal oxide surface. A method of molding a thermoset includes the steps of providing a mold having at least one face surface, forming a metal oxide coating on at least one face surface of the mold, providing an ionic release agent at the metal oxide surface, providing the acid or base conjugate of the release agent at the metal oxide surface, providing a thermoset resin mixture in the mold, and curing the resin mixture in the mold.
In one actual embodiment of the present invention, a metal oxide coating is deposited on a silica containing glass substrate by chemical or physical vapor deposition methods. The metal oxide is Al2O3. The release agent is an organic acid phosphate ester, and the resin mixture contains an isocyanate or epoxy.
In a second actual embodiment of the present invention, a method of using float glass having SnO2 on a surface is discolored, the method includes the step of providing an ionic release agent externally to the SnO2 surface, providing a mold formed from float glass and providing a urethane or an epoxy resin in the mold, and curing the resin.
In a third actual embodiment of the present invention, a system for molding a thermoset article includes a glass mold having a metal oxide surface, an applicator apparatus for applying a mold release to the metal oxide surface, and a washing apparatus for removing excess mold release from the metal oxide surface.
In a fourth actual embodiment, a plastic ophthalmic lens is produced by the method according to the invention.
In a fifth actual embodiment, a plastic optical filter is produced by the method according to the invention.
In a sixth actual embodiment, a filter replicated from a mold comprised of an optically engineered surface structure is made by the method according to the invention.
In a seventh actual embodiment, a reaction injection molding is made by the method according to the invention.
The present invention thus provides enhanced release properties for external and internal mold releases having an ionic character leading to less surface defects for many optical plastic consumer articles made by cell casting or reaction injection molding techniques.